Well, lo and behold, this week, I had a client from December come back and ask me to make some minor changes to their earlier project, (and, not inconsequentially, pay me a second time, the same amount as they did before). Not only did I still have all of their earlier files, (they didn't), but now I also had the original microphone that I had used back then. I took down the sound dampening panels, put things back they were a half year ago, and I achieved my goal of getting back the same (i.e., sub-par, but matching) sound.
Because I didn't have to re-record anything (everything) with a new mic, I was able to save myself about four or five hours of work, (on a fixed-price job), and I was able to cleanly splice together December and June recordings, oftentimes mid-sentence, without any discernible change. I must say, I was quite pleased with myself.